This invention relates to an axle assembly having dual wheels at each end of the axle in which one wheel on one axle end is permitted to rotate relative to another wheel on the same axle end. More specifically, the invention relates to a planetary drive with a differential supported between adjacent wheel hubs to achieve independent relative rotation during turns.
Drive axle assemblies have incorporated dual wheels on either end of the axle to increase the load bearing capability for heavy-duty vehicles. Typically the pair of wheels on each end of the axle assembly is secured together so that they rotate together at the same speed about an axis even during turns. Most off-highway vehicles, such as lift trucks, container handlers, front end loaders, etc., have duty cycles that include numerous turning maneuvers, which cause significant tire wear. Tire wear is a direct result of tire scrub, or drag, caused by wheels that are rigidly secured together for rotation at the same speed but which must travel different distances at the inside and outside of the turning radius. Tire wear and maintenance on heavy-duty off-highway vehicles due to scrub cost thousands of dollars annually per vehicle.
Additionally, tire scrub reduces vehicle stability during turning maneuvers. As a vehicle moves through a turn, tire scrub detracts from the net ground coefficient necessary for traction, braking, and side forces in turns. Tire scrub also causes premature wear of various wheel end components, such as wheel rims. The wheel rims can fail prematurely due to added stresses caused by repeated tire scrub. Finally, tire scrub increases fuel consumption and powertrain wear and tear.
For the above reasons, it would be desirable to reduce tire scrub by having a wheel end that allows adjacent wheel hubs to rotate independently relative to one other to allow each tire to rotate at different speeds through a turn.
A wheel end assembly having a dual wheel configuration includes independently rotating wheel hubs to reduce tire wear. In a disclosed embodiment of this invention, the wheel end includes a spindle and an axle shaft mounted for rotation relative to the spindle to define an axis of rotation. The wheel end further includes at least two wheel hubs supported for rotation about the axis. A planetary drive assembly drives the hubs to provide independent relative rotation. The planetary drive assembly includes an input member that is driven by the axle shaft and has a first output coupled to the first wheel hub and a second output coupled to the second wheel hub. A differential assembly drives the input member to achieve rotational speed differentiation.
In the preferred embodiment, the input member of the planetary drive assembly includes first and second sun gears driven by the differential assembly. The first output includes a first ring gear coupled to the first wheel hub and the second output includes a second ring gear coupled to the second wheel hub. A first set of planet gears are in meshing engagement with the first sun gear and the first ring gear and a second set of planet gears are in meshing engagement with the second sun gear and the second ring gear. The planet gears are supported on planet shafts that are fixed to the spindle.
In the preferred embodiment, the differential assembly includes side gears, differential pinions, and a differential spider. The spider is splined to and driven by the axle shaft. The differential pinions provide equal drive torque from the spider to the side gears. One side gear is mounted for rotation with the first sun gear and another side gear is mounted for rotation with the second sun gear. The differential pinions is are meshing engagement with the side gears to allow rotational speed differential between the first and second wheel hubs.
The subject invention offers a simple and compact design for independently rotating wheel hubs that significantly reduces tire wear resulting in less vehicle down time and which decreases overall maintenance cost. Further, because tire scrub is eliminated vehicle stability is improved and fuel economy is improved. These and other features of the present invention can be best understood from the following specifications and drawings, the following of which is a brief description.